1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring oscillation of the surface of a melt for use in growing a single crystal by Czochralski method.
2. Description of the Related Art
In single crystal growing apparatus which grows a single crystal by Czochralski method, an automatic control method has been established for the control of the growing operation conditions after forming a conical portion. However, manual control by a skilled operator is still necessary in the period for forming a neck portion, until the conical portion is formed after dipping of a seed crystal in the melt. This is because the formation of a neck portion requires highly delicate controls for the purpose of terminating dislocation at the crystal surface, such as growing of the crystal at a comparatively high velocity of 2 mm/min or greater while restricting the diameter of the crystal to 2 to 5 mm, and growing of the neck portion to a length which is 10 or more times greater than the diameter thereof while controlling the diameter at a precision of 0.5 mm or less in terms of the absolute value of the control offset. It is very difficult to obtain a desired form of the crystal without allowing the presence of dislocation in the transient period between the neck forming period and a subsequent increase in the crystal diameter. In fact, it is said that about 10% of such a manual control results in a failure even when it is done by those skilled in the art. More specifically, when the diameter of the crystal is reduced excessively, the lower end of the grown crystal is disconnected from the melt surface to disable further growth of the crystal or the strength of the crystal becomes too small to maintain the straight cylindrical portion of the crystal which is grown subsequently. Conversely, a too large crystal diameter hampers termination of dislocation, making it impossible to start the growth of the conical portion.
As shown in FIG. 1, a typical conventional apparatus for growing a single crystal has a a chamber 34 in which is disposed a gas-flow guiding sleeve 35 of a cylindrical cross-section arranged coaxially with the axis 10. In operation, argon gas flows through the gas-flow guiding sleeve 35 from the upper end to the lower end of the same. Since the argon gas is heated by the heat radiated from a melt 22, the flow of the argon gas tends to be disturbed. The flow of the argon gas is made to go outward through the gap between the lower end of the gas flow guiding sleeve 35 and the surface 22S of the melt. Part of the flow of the argon gas, however, collides with the surface of melt surface 22S to cause an oscillation of the melt surface 22S.
The oscillation of the melt surface 22S makes it difficult to continue the growth of the crystal particularly during the growth of the neck portion in which the crystal diameter is small, because the lower end of the grown crystal 32 tends to be disconnected from the melt surface 22S to disable further growth of the crystal.